1. Field of the Invention
This invention relates to a precoated corrosion-resistant steel sheet having a chromate undercoat and an organic topcoat. More particularly, it relates to such a duplex coated steel sheet which possesses good corrosion resistance and formability, can be finish coated by electrodeposition, and is preferably weldable by means of resistance welding so that it is highly suitable for use in automobile bodies.
2. Description of the Prior Art
Weldable precoated steel sheets which can be welded by electrical resistance welding have been increasingly used in automobile bodies in order to prevent them from rusting due to salt which is spread on roads for melting snow in snowy areas.
Typical weldable precoated steel sheets are Zincrometal (a registered trademark of Diamond Shamrock) and similar precoated steel sheets having a coating of a zinc-rich primer. Zincrometal comprises a steel sheet having an undercoat of a zinc-chromate solution (Dacromet, a registered trademark of Diamond Shamrock), and a topcoat of a zinc-rich epoxy resin-based primer (Zincromet, a registered trademark of Diamond Shamrock) and exhibits a significantly higher corrosion resistance than cold rolled steel sheets. Similar weldable precoated steel sheets called "Z-coat steel sheets" have an undercoat made by phosphate treatment and a topcoat of a zinc-rich primer such as Zincromet.
It is known that various additives may be incorporated in the zinc-chromate undercoat of Zincrometal. Such additives include reducing agents, metal chromates, oxides and hydroxides of an amphoteric metal, and hydrophilic colloids. See Japanese Patent Publications Nos. 47-6882(1972), 52-904(1977), and 52-4286(1977), and Japanese Patent Laid-Open Applications Nos. 49-74137(1974), 49-74138(1974), and 49-74139(1974).
In general, precoated steel sheets for use in automobile bodies or the like are required to have good formability, weldability, and corrosion resistance. In this connection, however, the properties, particularly the formability and corrosion resistance of the above-mentioned Zincrometal and Z-coat steel sheets are not satisfactory. This is because the zinc-rich primer used to form the topcoat of these precoated steel sheets contains a large amount of zinc powder or dust (hereinafter referred to as zinc powder) equal to around 50% on a volume basis or approximately 85% to 90% on a weight basis so that the topcoat films are brittle and tend to be readily peeled off during working or forming such as press forming. Such peeling or removal of the topcoat results in a significant loss of corrosion resistance of the precoated steel sheet. In addition, the removed pieces of the topcoat readily adhere to the die of the press machine, which may cause formation of flaws or scratches on the coated surfaces of precoated steel sheets being formed on the machine thereafter. Therefore, the die must be cleaned more frequently and the working efficiency is significantly decreased.
Another disadvantage of a zinc-rich primer is that the dry film thereof has a relatively large water permeability, which is also responsible for the propensity of its corrosion resistance to decrease. These problems, i.e., peeling of the coated film deterioration in corrosion resistance can be effectively alleviated by decreasing the content of zinc powder in the epoxy resin-based primer. However, this results in an increase in electrical resistance of the film, which makes it difficult or impossible to apply resistance welding to the precoated steel sheet.
In the above-mentioned precoated steel sheets, it is necessary to cure the topcoat of a zinc-rich primer by baking at a high temperature in the range of from 250.degree. to 280.degree. C., resulting in a loss of bake-hardenability of the base steel sheet if the base steel is of the bake-hardening type. The term "bake-hardening" used herein indicates that the yield stress of the steel is increased during baking of a finish coated applied, for example, by electrodeposition after press forming.
As another type of corrosion-resistant steel sheet, Japanese Patent Laid-Open Application No. 57-108292(1982) discloses a precoated steel sheet comprising a plated steel sheet with a Zn- or Al-based plating, the steel sheet having a chromate film formed on the plated surface and an organic composite coating formed on the chromate film. The organic composite coating comprises an organic water-soluble or water-dispersible resin such as an acrylic copolymer, epoxy resin, polyvinyl alcohol or starch and a silica sol (hydrophilic colloidal silica). The precoated steel sheet has improved corrosion resistance before and after finish coating and provides the finish coating with good adhesion.
It is also known that silica sol or colloidal silica may be incorporated into a chromate solution in order to improve the corrosion resistance of the chromated steel sheet and to increase the adhesion to a finish coating formed thereon. See, for example, Japanese Patent Publication No. 42-14050(1967).
It has been proposed to use a chromate solution in which a part of the hexavalent chromic acid has been reduced to trivalent chromium in order to decrease the solubility of the resulting chromate film, thereby improving the corrosion resistance of the steel sheet (Japanese Patent Publication No. 52-2851(1977)).
Japanese Patent Laid-Open Application No. 54-161549(1979) discloses a chromate solution which comprises partially reduced chromic acid and silica sol. A galvanized steel sheet treated with this solution has improved corrosion resistance due to the presence of Cr.sup.3+ and silica sol in the chromate film.
Japanese Patent Laid-Open Application No. 60-86281(1985) discloses a highly corrosion-resistant precoated steel sheet comprising a plated steel sheet having thereon a chromate undercoat layer and a topcoat layer of, e.g., a zinc-rich primer in which the chromate undercoat is formed from an aqueous suspension containing chromic acid, an iron phosphide powder, and optionally one or more substances selected from a dicarboxylic acid or a diol, zinc chromate or strontium chromate, oxides or hydroxides of zinc or strontium, and phosphoric acid.
Japanese Patent Laid-Open Application No. 61-239941(1986) discloses a weldable precoated steel sheet comprising a steel sheet plated with zinc or zinc base alloy, the steel sheet having a chromate film on the plated surface which is formed from an aqueous suspension containing chromic acid, an iron phosphide powder, and optionally a metal chromate, and a topcoat layer on the chromate film which is based on a polyhydroxypolyether resin formed by polycondensation of a mononuclear dihydric phenol or a mixture of a mononuclear dihydric phenol and a dinuclear dihydric phenol with an epihalohydrin.